The present invention relates to a rare earth fluoride phosphor giving UV luminescence, and a radiographic intensifying screen comprising the same.
Radiography (radiation image-forming system), in which a radiographic intensifying screen comprising a phosphor is used in combination with a silver halide photosensitive material, is generally known. In the system, the screen is placed on the front and/or the back surface of the photosensitive material. The phosphor in the screen thus placed absorbs the radiation (such as X-rays) having passed through an object or having radiated from an object, and emits visible luminescence. Since the photosensitive material is exposed to both of the radiation and the luminescence, the radiation image is efficiently recorded on the photosensitive material and reproduced as a visible image by development. As the phosphor, a phosphor emitting visible luminescence when irradiated with the radiation has been conventionally employed. Examples of the phosphors include a calcium tungstate phosphor and rare earth activated alkaline earth metal fluoride halide phosphors.
The radiographic intensifying screen has a basic structure composed of a.support and a phosphor layer provided thereon. If the phosphor layer is self-supporting, the support may be omitted. The phosphor layer usually comprises a binder and phosphor particles dispersed therein.
Recently, a radiographic intensifying screen comprising a phosphor giving UV luminescence around 400 nm as been proposed. In U.S. Pat. No. 4,225,653, for example, an intensifying screen comprising yttrium tantalate (YTaO4) phosphor or its Gd-, Nb- or Tb-containing phosphor is disclosed. The photosensitive material used in combination with the screen emitting UV luminescence does not need to contain visible dyes (e.g., spectral sensitizing dyes, and dyes for cutting cross-over), and therefore its processing solutions such as a developer and a fixer are not polluted with the dyes oozing out of the photosensitive material in the course of the processing. Consequently, deposition of stain on the automatic processing machine can be reduced, and the maintenance of the developing system can be remarkably simplified. Further, since the photosensitive material is not stained with residual dyes, the development process can be favorably carried out in a relatively short time with a small amount of replenisher.
As the phosphor giving UV luminescence, lanthanide fluorides (e.g., CeF3) and rare earth activated lanthanide fluorides (e.g., LaF3:Ce, and GdF3:Ce,Tb) are known. These phosphors are practically employed, for example, as scintillator.
The present inventor studied on phosphors giving UV luminescence suitable for the above radiation image-forming system, and found the following facts. Rare earth fluoride phosphors and their trivalent metal activated phosphors emit UV luminescence in a shorter wavelength region than yttrium tantalate (YTaO4) phosphor, and hence they are very suitably used in combination with a silver chloride photographic emulsion having an intense absorption band at the shorter UV wavelength region. Accordingly, in addition to the above-described advantages of the system employing UV luminescence, the screen comprising those phosphors can substantially enhance the sensitivity of the photosensitive material so as to improve diagnostic performance without increasing the radiation dose.
As described above, the rare earth fluoride phosphors and their trivalent metal activated phosphors emit UV luminescence in a wavelength region suitable for the radiation image-forming system employing UV luminescence. However, the intensity of the luminescence is not fully satisfactory.
It is an object of the present invention to provide a rare earth fluoride phosphor emitting UV luminescence of improved intensity.
It is another object of the invention to provide a radiographic intensifying screen of high sensitivity giving UV luminescence in the wavelength region of 300 to 350 nm.
The present invention resides in a rare earth fluoride phosphor having the following formula (I):
(Gd1xe2x88x92xLnx)F3xc2x7aMXxe2x80x83xe2x80x83(I)
in which Ln is at least one rare earth element selected from the group consisting of Y, La and Lu; M is at least one alkali metal element selected from the group consisting of Na, K, Cs and Rb; X is at least one halogen selected from the group consisting of F, Cl, Br and I; and a and x are numbers satisfying the conditions of 0 less than axe2x89xa60.03 and 0xe2x89xa6xxe2x89xa60.1, respectively.
The invention also resides in a trivalent metal activated rare earth fluoride phosphor having the following formula (II):
LnF3xc2x7aMX:yAxe2x80x83xe2x80x83(II)
in which Ln is at least one rare earth element selected from the group consisting of Y, La, Gd and Lu; M is at least one alkali metal element selected from the group consisting of Na, K, Cs and Rb; X is at least one halogen selected from the group consisting of F, Cl, Br and I; A is at least one trivalent metal element selected from the group consisting of Ce, Tb and Bi; and a and y are numbers satisfying the conditions of 0 less than axe2x89xa60.03 and 0 less than y xe2x89xa60.1, respectively.
The invention further resides in a radiographic intensifying screen comprising the rare earth fluoride phosphor of the above formula (I) or the trivalent metal activated rare earth fluoride phosphor of the above formula (II).